As an example of products that are manufactured by welding a metal member to another metal member including permanent magnet members, there are rotors of an electric rotating machine. For example, in the case of rotors of a vehicle AC generator, the following rotor is disclosed. In this rotor, a pair of field cores (pole cores), which are abutted on a rotary shaft, have pawl-shaped magnet poles shaped to axially extend at the perimeter, and to be engaged with each other; permanent magnet members are mounted in gaps between the circumferentially opposed side faces of the adjacent pawl-shaped magnet poles; and cooling fans for suppressing the rise of temperature of a power generator are fixed at the axial front and back of this field core fixed by, e.g., welding (for example, refer to Patent Document 1).
Furthermore, as welding sets that weld cooling fans to a field core, for example, a welding set of welding cooling fans to a magnetic pole of a vehicle charging generator is disclosed. In this welding set, a shaft of a rotor is directed in a vertical direction; cooling fans are put on the back faces of magnetic poles; electrodes for welding are pressed from outside of the cooling fans; current is carried in sequence of the upper-side electrode, the upper-side cooling fan, the upper-side magnetic pole, the lower-side magnetic pole, the lower-side cooling fan, and the lower-side electrode, while pressing the electrodes located above and below to be welded. A welding current made to flow through a weld area at this time is generally DC (for example, refer to Patent Document 2).
Upon welding cooling fans of a rotor as shown in Patent Document 1 to pole cores, welding with the use of a welding set as shown in the Patent Document 2 undergoes the following manufacturing process. In this process, cooling fans are brought in contact at a predetermined position of axial end faces of pole cores after permanent magnet members have been mounted onto the pole cores; electrodes are held with one electrode electrically connected to one cooling fan, and the other electrode electrically connected to the other cooling fan located on the opposite side of the pole cores; and, for example, a pulsed current of dozens kA at peak value is carried between both of the electrodes to make the resistance welding. At this time, a welding current will flow through axially in the vicinity of the shaft of a rotor. As described above, since there are mounted permanent magnet members in gaps between the opposed faces of the pawl-shaped magnetic poles, in the case where a welding current flowing through a plurality of weld points is carried concentratedly, a magnetic filed that is generated by this large current will be in a magnetic field intensity large enough to magnetize permanent magnet members, and the permanent magnet members will be magnetized circumferentially in the same polarity. A purpose of the use of permanent magnet members of a rotor is to counteract magnetic fluxes being leaked in gaps between the adjacent pawl-shaped magnetic poles, so that it is necessary that an elongated permanent magnet member be polarized in different magnetic poles at both sides of a thickness direction. However, there arises such a trouble that half of permanent magnet members will be polarized in a direction opposite to the direction to be originally polarized under the influence of a welding current as described above. A magnetization at this time, through experiments made by inventors, is confirmed to exceed 80% on average when letting a full magnetization 100%.
Normally, it is desirable to employ, as magnets for use in a rotor of electric rotating machines, magnets having a high residual magnetic flux density for the purpose of improving a starting torque, as well as having a high coercive force so as to be resistant to operating conditions at high temperature (e.g., neodymium magnets). In the case of magnets of high coercive force, however, once the magnets have been polarized in the opposite direction, a stronger magnetic field will be required as compared with the first-time polarization in order to make the forward polarization next, resulting in the insufficient polarization in the normal polarization process. Furthermore, since iron power, which is produced at the time of machining, sticks to the polarized magnets not to be removed, and the iron powder remains as foreign substance after assembling as an electric rotating machine, a problem exits in the occurrence of such a trouble as coils are damaged to be short-circuited.
Patent Document 1: the Japanese Patent Publication (unexamined) No. 56616/1993 (pages 2-3, FIG. 1)
Patent Document 2: the Japanese Patent Publication (unexamined) No. 205757/1997 (page 2, FIG. 2)